Currently, a non-aqueous electrolyte secondary battery including a lithium ion secondary battery, which is used for a mobile device such as a mobile phone, is available as a commercial product. The non-aqueous electrolyte secondary battery generally has a constitution that a positive electrode having a positive electrode active substance or the like coated on a current collector and a negative electrode having a negative electrode active substance or the like coated on a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution or a non-aqueous electrolyte gel is maintained within a separator. According to absorption and desorption of ions such as lithium ions on an electrode active substance, charging and discharging reactions of a battery occur.
In recent years, it is desired to reduce the amount of carbon dioxide in order to cope with the global warming. As such, a non-aqueous electrolyte secondary battery having small environmental burden has been used not only for a mobile device or the like but also for a power source device of an electric vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), and a fuel cell vehicle.
As the non-aqueous electrolyte secondary battery for application to an electric vehicle, it is required to have high output and high capacity. As a positive electrode active substance used for the positive electrode of a non-aqueous electrolyte secondary battery for an electric vehicle, a lithium cobalt-based composite oxide, which is a layered composite oxide, has been already widely used since it can provide high voltage at the level of 4 V and has high energy density. However, due to resource scarcity, cobalt as a raw material is expensive, and considering the possibility of having dramatic demand in future, it is not stable in terms of supply of a raw material. There is also a possibility of having an increase in the raw material cost of cobalt. Accordingly, a composite oxide having less cobalt content ratio is desired.
A spinel type lithium manganese composite oxide (LiMn2O4) has a spinel structure and it functions as a positive electrode material of 4 V grade according to the composition with λ-MnO2. By having a three dimensional host structure which is different from a layered structure of LiCoO2 or the like, most of the theoretical capacity of the spinel type lithium manganese composite oxide is usable and it is expected to have excellent cycle characteristics.
However, with a lithium ion secondary battery in which the spinel type lithium manganese composite oxide is used as a positive electrode material, it is actually impossible to avoid capacity deterioration which exhibits a gradual decrease in capacity according to repeated charge and discharge. As such, there has been a big problem for putting it to practical use.
As a technique for solving the problem of capacity deterioration of a spinel type lithium manganese composite oxide, in JP 2000-77071 A, for example, a technique of further using, as a positive electrode material, a lithium nickel-based composite oxide (LiNiO2, Li2NiO2, LiNi2O4, Li2Ni2O4, LiNi1-xMxO2, or the like) with a predetermined specific surface area in addition to a spinel type lithium manganese composite oxide is disclosed. According to JP 2000-77071 A, it is described that, by having such constitution, dissolution of Mn from the spinel type lithium manganese composite oxide or a change in Li concentration in an electrolyte solution is suppressed, and as a result, a non-aqueous electrolyte secondary battery with highly improved charge and discharge cycle characteristics (in particular, charge and discharge service life at high temperature) can be provided.